Southwest Airlines has been sued by the U.S. government for reportedly failing to pay a fine levied when the company did not comply with regulations related to Boeing 737 jet repairs.

The lawsuit, filed by the Justice Department in the district court of Washington, seeks to enforce a $12 million civil penalty proposed by the U.S. Federal Aviation Authority (FAA) in July, Reuters reported.

The FAA investigated 44 Southwest-operated Boeing 737 aircraft serviced by Aviation Technical Services Inc, an aircraft repair firm, between 2006 and 2009 and found that they were improperly maintained, according to court documents filed on Monday.

"We dispute the FAA's allegations and look forward to the opportunity to vigorously defend Southwest's record in a court of law," Southwest spokesman Chris Mainz said in an email to Reuters.

Southwest has 21 days to respond to the lawsuit, according to the court documents.